Recently, a nonaqueous electrolyte battery such as a lithium ion secondary battery has been developed as a high energy-density battery. The nonaqueous electrolyte battery is anticipated as a power source for vehicles such as hybrid automobiles and electric cars. Attention is also paid on them as a battery for fixed power sources for applications such as averaging out the amount of electricity consumption between day and night or for smart grids. Therefore, the nonaqueous electrolyte battery is demanded to have other good performances such as rapid charge-and-discharge performances and long-term reliability, as well. A nonaqueous electrolyte battery capable of rapid charge and discharge has the benefit that charging time is remarkably short, and is able to improve motive performances in hybrid automobiles. Furthermore, the battery can also efficiently recover regenerative energy from power of the vehicle.
Rapid charge-and-discharge becomes possible by rapid migration of electrons and lithium ions between the positive electrode and the negative electrode. However, when a battery using a carbon-based negative electrode is repeatedly subjected to rapid charge-and-discharge, dendrite of metallic lithium may sometimes precipitate on the electrode. Dendrites cause internal short circuits, and as a result raise concern that heat generation and/or fires may occur
In light of this, a battery using a metal composite oxide as a negative electrode active material in place of a carbonaceous material has been developed. In particular, in a battery using titanium oxide as the negative electrode active material, rapid charge-and-discharge can be stably performed. Such a battery also has a longer life than those using a negative electrode with carbonaceous material.
However, compared to carbonaceous materials, oxides of titanium have a higher potential (is more noble) relative to metallic lithium. Furthermore, oxides of titanium have a lower capacity per weight. Therefore, a battery using an oxide of titanium as the negative electrode active material has a problem that the energy density is lower.
For example, the potential of the electrode using an oxide of titanium is about 1.5 V relative to metallic lithium and is higher (more noble) than that of the negative electrode with carbonaceous material. The potential of an oxide of titanium arises from the redox reaction between Ti3+ and Ti4+ upon electrochemical insertion and extraction of lithium, and is therefore electrochemically limited. There also is the fact that at a high electrode potential of about 1.5 V, rapid charge-and-discharge of lithium ions can be performed stably. It is therefore practically difficult to drop the potential of the electrode in order to improve the energy density.
On the other hand, considering the capacity per unit weight, the theoretical capacity of lithium titanate (anatase structure) is about 165 mAh/g, and the theoretical capacity of a lithium-titanium composite oxide such as Li4Ti5O12 is about 180 mAh/g. On the other hand, the theoretical capacity of a general graphite based electrode material is 385 mAh/g and greater. Therefore, the capacity density of an oxide of titanium is significantly lower than that of the carbon based negative electrode material. This is due to there being only a small number of lithium-insertion sites in the crystal structure, and lithium tending to be stabilized in the structure, and thus, substantial capacity being reduced.
In view of such circumstances, a new electrode material including titanium (Ti) and niobium (Nb) has been examined. Such materials are expected to have high charge-and-discharge capacities. In particular, a composite oxide represented by TiNb2O7 has a theoretical capacity exceeding 380 mAh/g; however, the substantial capacity of an electrode of TiNb2O7 is about low as 260 mAh/g, and has the problem that the charge-and-discharge life is short.